(1). Field of the Invention
This invention relates, in general, to a silk screen printing machine, and more particularly to a silk screen printing machine comprising an annular-shaped rotatable transport member which is automatically indexed from station-to-station in a multicolor silk screen printing process. Even more specifically, the present invention relates to apparatus for, and a method of, automatically adjusting the position of the screen frame holder, hence the screen frame, mounted to each of the silk screen print heads in a silk-screen printing machine relative to each of a plurality of piece parts located on the transport member to be printed, to maintain accurate print-to-print or color-to-color registration. The present invention particularly relates to a means for, and method of, automatic adjustment of the screen frames mounted to the print heads in the x-axis to increase the accuracy of the print-to-print/color-to-color registration on a compact disc in a multicolor silk screen printing process.
(2). Description of the Prior Art
Flat substrates or piece parts, e.g., compact discs, are commonly printed automatically with graphics by means of a silk screen printing machine comprising, in its basic aspects, an annular-shaped, horizontally disposed, rotatable transport member and a plurality of printing heads located in a plane above the top planar surface of the transport member on a fixed support member. A plurality of tooling fixtures, each for supporting a compact disc for printing, are provided on the top planar surface of the transport member, these being equally spaced apart and defining a like plurality of positions to which the annular-shaped transport member is indexed.
In general, the compact discs are loaded one-at-a-time onto each of the plurality of tooling fixtures, each compact disc being registered in a precise location on the tooling fixture for printing, and vacuum being applied to the bottom of the compact disc to maintain it in the registered position. Once a compact disc is registered on the tooling fixture, the dial then indexes, in turn, to each of the defined positions. At each of the positions where there is located a printing head, the top surface of the compact disc receives a layer of ink in the form of text or graphics.
A silk screen print head basically comprises a squeegee assembly and a silk screen frame holder, the silk screen comprising a screen frame to which is adhesively secured a "silk screen," e.g. a stretched polyester fabric of plain weave. The images or graphics to be printed are provided in an exposed photosensitive emulsion coated on the silk screen in outline form. Thus, at each of the printing stations there is provided a "stencil" having a slightly different outline for the application of the color of ink at that station to the compact discs. The compact discs, on being indexed, must each stop exactly at the position defined by each print head; otherwise, the colors of ink applied to the compact disc surface may be out of acceptable registration with one another.
The dial or annular-shaped rotatable transport member in a silk screen printing machine is commonly one of two types of cam driven indexing systems., i.e., a continuous feed cam or a mechanical cam. A continuous feed cam requires a servo motor drive and very high precision drive components due to the fact that whenever the transport member is indexed the whole drive train is started and stopped.
When a compact disc is loaded onto a tooling fixture, it indexes a multiple number of times before being off-loaded, the number of times that the compact disc is indexed depending upon the number of positions defined by the tooling fixtures provided on the transport member. Thus, where thirty five tooling fixtures, as is now common, are provided on the rotatable transport member, each compact disc is indexed to thirty five positions via, for example, five screen print heads each applying a different color of ink, before being off-loaded. Each time the transport member indexes from one position to another, there can be a small rotational error. To reduce the rotational errors causing misregistration of color-to-color overlay and to provide that the color-to-color overlay is within specification, the rotatable transport member is calibrated. In general, the calibration involves first determining the offset error for each of the thirty five tooling fixtures. Next, a new cam value is determined, i.e., a location for each of the thirty five positions of the cam that provides acceptable registration for each tooling fixture. This is made possible because of the high precision components used and a servo motor drive system linked to an output encoder that has the capability of changing the final stop position of each index. To verify whether the cam now places the compact discs under the print heads at the right location to provide color-to-color overlay within specifications, the above procedure is repeated, i.e., the offsets errors are again determined. The calibration of the rotatable transport member in this manner typically takes several iterations to achieve satisfactory results.
Mechanical indexing type cam driven systems tend to provide good repeatability, i.e., the annular-shaped rotatable dial stops at the same location for a position each time it comes to the same relative position. Thus, for example, tooling fixture #1 will always stop at the same location in the circular-defined path of travel when it is indexed to printing station #1. Nevertheless, the accuracy, i.e., the ability of the dial to stop at equal intervals for each index, may not meet the specifications set for print-to-print/color-to-color overlay. For example, tooling fixture #2, when indexed to printing station #1 may stop at a different location than did tooling fixture #1. The same is true for each of the tooling fixtures located on the transport member being indexed to printing station #1. In other words, the compact discs may not all line up on the x-axis with the position defined by a print head for printing. There is an offset from the location on the x-axis where the compact disc should be for printing relative to the print head caused by the inaccuracies of the indexing system.
Nevertheless, with either a continuous feed cam or indexing cam driven system the best color-to-color registration that can be achieved in a silk screen printing process, e.g., with six color printing is .+-.0.002". With the silk screen printing industry constantly requiring better color-to-color registration, this degree of registration is not satisfactory.
Thus, there is a real need for automated silk screen printing apparatus comprising an indexing rotatable transport member wherein more accurate color-to-color registration can be maintained in a multicolor printing process.